Generally, the present invention relates to fiber optic communication systems, and particularly to laser sources for pumping fiber amplifiers.
In many optical systems, such as pumps in optical communication systems, fiber gyros, control channels in optical amplifiers, sensors, and inteferometers, it is desirable to have a depolarized source to reduce polarization effects. For example, optical communications systems are increasingly using fiber systems that include fiber amplifiers for amplifying the optical communications signal. In particular, Raman amplifiers are becoming increasingly used for distributed or remote amplification. These amplifiers are useful because they may be implemented in the normal fiber that carries the optical communications signal, and do not require the insertion of a special type of fiber, as is the case with a rare earth-doped fiber amplifier. As a result, systems may be designed where amplification occurs in the transmission link itself, allowing greater transmission distances between amplifiers, higher receiver sensitivities, and lower transmission powers. Lower transmission powers have the added advantage of reducing nonlinear effects in the fiber. Furthermore, the wavelength at which the fiber Raman amplifier operates is determined by the wavelength of the pump light, unlike a rare earth-doped fiber amplifier whose gain bandwidth is limited by the rare earth species doped in the fiber. Raman amplification is also a very low noise process, limited primarily at low powers by pump noise and quantum noise effects.
One particular concern with Raman amplification is the polarization dependence of the amplification process. The Raman gain coefficient when the pump light is polarized parallel to the signal polarization is about an order of magnitude greater than when the pump polarization is orthogonal to the signal polarization.
A long-haul single mode optical fiber is usually non-polarization preserving due to its lower cost and superior performance compared to polarization maintaining fiber. Therefore, the polarizations of the pump and signal to change relative to each other as they propagate along the length of the fiber amplifier. In practice, Raman amplifiers are long, up to several kilometers in length, which permits sufficient variation in the relative polarizations of the signal and the pump that the polarization-dependent gain effect is averaged out.
However, there remains a possibility that the pump and signal maintain the same relative polarizations for a significant length of the fiber amplifier. In such situations, the amplifier gain may be strongly dependent on the relative polarizations of the signal and the pump. This leads to uncertainty in amplifier performance, which results in increased errors in signal detection or increased system margin requirements.
Therefore, there is a need for fiber amplifier systems that have a reduced dependency on the relative polarizations of the signal and pump beams, so that amplifier performance can be more predictable and reliable.
A semiconductor pump laser uses a depolarizer to depolarize the pump light entering the fiber amplifier. The depolarized light source is useful for reducing polarization dependent gain of fiber amplifiers.
In one embodiment of the invention, a fiber communications system includes a coherence-collapsed laser source including a semiconductor laser element emitting a polarized output; and a depolarizer coupled via a polarization maintaining path to the laser element to receive the polarized output and configured to depolarize the polarized output from the laser element to produce a depolarized laser signal.
In another embodiment of the invention, an optical fiber system includes coherence collapsed light emitting means for emitting polarized light and light depolarizing means for depolarizing the polarized light received from the light emitting means via a polarization preserving path. Fiber amplifying means are coupled to receive depolarized light from the light depolarizing means.
Another embodiment of the invention includes a pump light source for pumping a fiber amplifier. The pump light source includes at least two coherence collapsed lasers emitting pump light from at least two respective outputs and at least one depolarizer disposed to depolarize pump light from the at least two coherence collapsed lasers. A combiner combines light from the at least two coherence collapsed lasers, the combiner having at least one output fiber couplable to one or more active fiber devices.
Another embodiment of the invention is directed to a laser source including a semiconductor laser element emitting a polarized output. A wavelength selective reflector is disposed to feed a portion of the output back to the laser element, whereby a coherence length of the polarized output is reduced. A depolarizer is disposed to depolarize the output from the laser element.
Another embodiment of the invention is directed to a laser system that includes a laser source having a semiconductor laser element emitting a polarized output having a coherence length of less than 30 cm and a depolarizer disposed to depolarize the polarized output from the laser element.
Another embodiment of the invention is directed to a laser system that includes a semiconductor laser element emitting a polarized output, and a polarization maintaining fiber coupled to receive the polarized output from the semiconductor laser element, a polarization axis of the polarization maintaining fiber being oriented at approximately 45xc2x0 relative to a polarization direction of the polarized output from the laser element.
Another embodiment of the invention is directed to a laser system that includes a semiconductor laser element producing an output signal. The laser element is operable in a first operating mode with the output signal having a first coherence length and in a second operating mode with the output signal having a second coherence length less than the first coherence length. A depolarizer is disposed on a light path from the laser element such that light having the second coherence length is substantially depolarized while light having the first coherence length is substantially unchanged in degree of polarization.
Another embodiment of the invention is a fiber depolarizer that includes an Nxc3x97M polarization preserving coupler having N inputs and M outputs, N and M being at least 2; an input to the depolarizer at a first coupler input and output port from the depolarizer at a first coupler output. A polarization-controlling optical path is coupled between a second coupler output and a second coupler input. A polarization of light exiting the Nxc3x97M coupler through the second coupler output is substantially orthogonal to a polarization of light entering the Nxc3x97M coupler through the second coupler input.